This invention concerns the improvement of fish culture through control of stickleback by treating its aquatic habitats with chemicals that contain the azide radical (N.sub.3 --) and dissociate to form free azide radicals in water. An example of one such chemical is sodium azide (NaN.sub.3).
The selective eradication of pest fish allows more food for the desirable species. This permits the desirable fishes to grow faster and become less vulnerable to predation. Depensatory mortality of stickleback by unharmed desirable predators slows re-infestation of stickleback. Selective eradication does not destroy the gene pools of indigenous stocks of desirable fishes that are very difficult to replace.
On the other hand, the use of general toxicants kills both desirable and pest species of fish. After poisoning all species, stickleback populations would grow faster than salmonid populations and thus gain the advantage in competition for food. The stickleback population would grow faster due to shorter generation time and lack of predators. Native stocks of salmonids would be destroyed and difficult to re-establish.
The stickleback, or target fish, and a salmonid, a desirable fish, were exposed together to low concentrations of single toxicants in 24-hour bioassays at 15 degrees Celsius. The order and time of death of each species of fish were noted. The results this procedure indicated that azides are extremely selective for stickleback and harmless to salmonids.
Five species of the stickleback family exist in North America of which the threespine stickleback (Gasterosteus aculeatus) is the most notorious pest. The threespine stickleback is almost circumpolar in distribution and is normally found in marine, brackish and fresh waters of the northern hemisphere.
The stickleback has a high reproductive potential; as many as 600 eggs have been found in one nest. The fact that the male guards the nest and newly hatched young enhances juvenile survival. The stickleback has the advantage of a much shorter generation time (one to two years) than most salmon and trout (two to seven years). This permits the stickleback to increase their numbers at a faster rate than salmonids and become extremely abundant. Furthermore, stickleback have an exceptionally high rate of survival since they take advantage of inshore areas in which large predators are not commonly found.
Stickleback greatly reduces the populations of food and game fishes, usually species of salmon and trout, with which they compete. Furthermore, it is a successful competitor for limited food supplies and attacks juvenile salmon and trout. The result is that trout and salmon are scarce where stickleback populations abound. As stickleback have no value as a sport or commercial fish, they are considered a pest fish and constitute an aggravating problem wherever they exist with desirable species of fish.
Spot poisoning of stickleback with general toxicants like rotenone and toxaphene has been ineffective in controlling stickleback numbers since any benefits are very temporary and such methods are often carried out at the expense of killing natural stocks of desirable species.
We have found that azide compounds are outstanding toxicants to stickleback in dilutions which are harmeless to desirable aquatic species.